1. Field
Embodiments relate to a semiconductor memory device. More particularly, embodiments relate to a nonvolatile memory device calibrating a program start voltage and a programming method thereof.
2. Description of the Related Art
Semiconductor memory devices are devices that are able to store data and read data when it is necessary. Semiconductor memory devices are generally classified into Random Access Memory (RAM) devices and Read Only Memory (ROM) devices. RAM devices are volatile memory devices in which stored data may be lost when a power supply is turned off. ROM devices are nonvolatile memory devices in which stored data may be maintained, e.g., not lost, even when a power supply is turned off. RAM devices include Dynamic RAM (DRAM) devices, Static RAM (SRAM) devices, etc. ROM devices include Programmable ROM (PROM) devices, Erasable PROM (EPROM) devices, Electrically EPROM (EEPROM) devices, flash memory devices, etc. Flash memory devices are generally classified into NOR and NAND flash memory devices. An integration degree of a NAND flash memory device is higher than that of a NOR flash memory device.
A nonvolatile memory device such as the NAND flash memory device stores data using memory cell transistors. Each of the memory cell transistors may include a charge storage layer such as a floating gate or a charge trap layer. Charge may be accumulated or trapped in the charge storage layer when a program voltage having a predetermined level is applied to a control gate of the memory cell transistor. Accumulated or trapped charge may effectively change a threshold voltage of the memory cell transistor. As a result, the memory cell transistor may store data.
As a result of process variations, memory cell transistors may have different characteristics from each other. That is, e.g., some of the memory cell transistors may be programmed faster than others of the memory cell transistors. A memory cell transistor that is programmed faster than normal memory cell transistors is called a “tail-bit memory cell transistor”. If the same program voltage is applied to memory cell transistors in a selected row, tail-bit memory cell transistors may be over-programmed to have higher threshold voltages than the normal memory cell transistors. Thus, tail-bit memory cell transistors may operate as off-cell transistors, which are always maintained at a turn-off state, and may cause read errors of the nonvolatile memory device.